1. Field of the Invention
The present invention relates to a wound-core golf ball with a polyurethane cover, which exhibits a long flight distance, stops easily due to a higher spin rate even if it is shot in the rough or in the rain, and imparts a good shot feeling like a Balata-covered golf ball to the golfer.
2. Description of the Related Art
A wound-core golf ball having a xe2x80x9cBalataxe2x80x9d cover is widely used by high-level golfers and professional golfers, because Balata cover is superior in feel and control. Herein, xe2x80x9cfeelxe2x80x9d is the overall sensation to the golfer when the golf ball is hit, and xe2x80x9ccontrolxe2x80x9d is to impart the motion such as back spin to the golf ball. However, the golf ball having a Balata cover is expensive due to a complicated manufacturing process and is inferior in durability and cut resistance.
For this reason, various cover stocks have recently been proposed in an attempt to replace the Balata cover.
For instance, a golf ball having an ionomer resin is disclosed in Japanese Unexamined Patent Publication No. H01-308577. The ionomer resin cover is less expensive and has a superior durability than the Balata cover. However, it is pointed out that the ionomer resin cover is poor in feel.
U.S. Pat. No. 4,123,061 discloses a golf ball made from a polyurethane prepolymer including toluene diisocyanate compound and 3,3xe2x80x2-dicholoro-4,4xe2x80x2-diaminodiphenylmethane as a curing agent. Golf balls with the polyurethane cover are known to have high impact resilience and to impart a good shot feeling as do Balat-covered golf balls.
As a curing agent for polyurethane prepolymer, aromatic polyamine compound is preferably used, because the heat resistance and mechanical properties of the resultant product are good. Especially, 3,3xe2x80x2-dichloro-4,4xe2x80x2-diaminodiphenylmethane and its derivatives are preferably used as a curing agent by the skilled artisan.
The wound-core golf ball with a polyurethane cover is formed by coating a composition including isocyanate group terminated polyurethane prepolymer and aromatic polyamine compound around the wound-core and then curing the composition. However, the following problems are pointed out in a process for mixing and curing the composition.
In the case that the aromatic polyamine compound has low reactivity to isocyanate group, it is inevitable that the curing time becomes longer, which results in lowering the productivity. When the curing reaction is not enough, the properties of the resultant polyurethane cover is also lowered. On the other hand, in the case that the reactivity to isocyanate group is too high, the polyurethane prepolymer increase in viscosity or become gelatinized during mixing.
Most of aromatic amine compounds have so high cohesion strength that they are crystalline at a room temperature. Thus, these aromatic amine compounds must be melted by heating them at least their melting point prior to use. Especially, 3,3xe2x80x2-dichloro-4,4xe2x80x2-diaminodiphenylmethane and its derivatives, which are preferably used as a curing agent for producing polyurethane, have the melting point of about 100xc2x0 C., thus these curing agents usually need to be melted by heating about 120xc2x0 C. for mixing and curing. For this reason, the curing reaction is promoted by the heat of these curing agents, resulting in increase in the viscosity or gelation of the polyurethane prepolymer. Further, the rubber thread layer is deteriorated by the heat when the curing reaction is conducted at 120xc2x0 C. If the curing temperature and mixing temperature is lowered to avoid the heat deterioration of the rubber thread layer , deposit generates in the mixture of the polyurethane prepolymer and the curing agent. When the deposit generates, it is impossible to make the polyurethane cover due to the heterogeneous curing reaction.
According to the present invention, the wound-core golf ball with the polyurethane cover which is formed by curing a composition including isocyanate group terminated polyurethane prepolymer and aromatic amine compound homogeneously is obtained. An object of the present invention is to provide a wound-core golf ball with the polyurethane cover, which does not only overcome the above problems such as increase in the viscosity and gelation of the polyurethane prepolymer or generation of the deposit but also is excellent in terms of shot feeling, controllability and flight distance.
The inventor of the present invention has found that the deposit does not generate in the mixture of the isocyanate group terminated polyurethane prepolymer and aromatic amine compound even at low temperature if the isocyanate group terminated polyurethane prepolymer contains residual polyisocyanate monomer in an amount of not more than 0.1 weight % and has achieved the present invention.
Namely, the present invention provides a wound-core golf ball comprising
a center;
a rubber thread layer covering the center; and
a polyurethane cover covering the rubber thread layer, wherein the polyurethane cover is formed by curing a composition including an isocyanate group terminated polyurethane prepolymer containing a residual polyisocyanate monomer in an amount of not more than 0.1 mass %; and an aromatic polyamine compound.